Reachability from the Capital CodeForces - 999E(强连通分量 缩点 入度为0的点)

#include #define mem(a, b) memset(a, b, sizeof(a))
#define rap(i, a, n) for(int i=a; i#define rep(i, a, n) for(int i=a; i#define lap(i, a, n) for(int i=n; i>=a; i--)
#define lep(i, a, n) for(int i=n; i>a; i--)
#define rd(a) scanf("%d", &a)
#define rlld(a) scanf("%lld", &a)
#define rc(a) scanf("%c", &a)
#define rs(a) scanf("%s", a)
#define pd(a) printf("%d\n", a);
#define plld(a) printf("%lld\n", a);
#define pc(a) printf("%c\n", a);
#define ps(a) printf("%s\n", a);
#define MOD 2018
#define LL long long
#define ULL unsigned long long
using namespace std;
const int maxn = 10010, INF = 0x7fffffff;
vectorint> G[maxn];
int pre[maxn], lowlink[maxn], sccno[maxn], dfs_clock, scc_cnt;
int in[maxn];
stackint> S;
int n, m, s;
void dfs(int u)
{
    pre[u] = lowlink[u] = ++dfs_clock;
    S.push(u);
    for(int i=0; i)
    {
        int v = G[u][i];
        if(!pre[v])
        {
            dfs(v);
            lowlink[u] = min(lowlink[u], lowlink[v]);
        }
        else if(!sccno[v])
            lowlink[u] = min(lowlink[u], pre[v]);
    }
    if(lowlink[u] == pre[u])
    {
        scc_cnt++;
        for(;;)
        {
            int x = S.top(); S.pop();
            sccno[x] = scc_cnt;
            if(x == u) break;
        }
    }
}
void init()
{
    dfs_clock = scc_cnt = 0;
    mem(sccno, 0);
    mem(pre, 0);
}

int main()
{
    init();
    int u, v;
    cin>> n >> m >> s;
    for(int i=0; i)
    {
        cin>> u >> v;
        G[u].push_back(v);
    }
    for(int i = 1; i)
        if(!pre[i]) dfs(i);
   // cout
    for(int i=1; i)
        for(int j=0; j)
            if(sccno[i] != sccno[G[i][j]])
                in[sccno[G[i][j]]]++;
    int cnt = 0;
    if(in[sccno[s]] == 0)
        cnt--;
    for(int i=1; i)
    {
        if(in[i] == 0)
            cnt++;
    }
    coutendl;

    return 0;
}